In these years, there have been needs for reductions in weight and thickness of personal computers and TV sets. According to such needs, flat-panel displays such as liquid crystal displays (LCD) have been developed, as substitutes for cathode-ray tubes (CRT).
An LCD is a display that applies an electric field to a liquid crystal layer having anisotropic permittivity (anisotropic dielectric constant), the liquid crystal layer being interposed between two substrates. By adjusting the electric field, the LCD controls a volume of light passing through the substrates so as to obtain desired image signals. The LCD is well known among other flat-panel displays convenient for carriage. Especially, a thin film transistor (TFT) LCD utilizing a TFT as a switching element is predominantly used.
With reference to FIGS. 10 through 14, a conventional liquid crystal display is described. FIG. 10 is a block diagram illustrating a schematic structure of the conventional liquid crystal display. FIG. 10 shows an LCD controller 1, a liquid crystal display panel 2, a signal line drive circuit 3, a scan line drive circuit 4, a reference gradation voltage generation section 5, a backlight 6, and a backlight driving inverter circuit 7.
Image data is input in the LCD controller 1, as gradation data D11 and synchronous data D12. The gradation data D11 is RGB signals, for example. The synchronous data D12 includes a vertical synchronizing signal, a horizontal synchronizing signal, a data enable signal (DE), a clock and others. Based on the input gradation data D11 and synchronous data D12, the LCD controller 1 generates not only gradation data D13 and a signal-side control signal D14 to be output to the signal line drive circuit 3, but also a scan line control signal D15 to be output to the scan line drive circuit 4. With this arrangement, the LCD controller 1 controls image display on the liquid crystal display panel 2.
With reference to FIG. 11, a structure of the liquid crystal display panel 2 (active matrix type LCD) is described. The liquid crystal display panel 2 has a first and a second glass substrates (not shown). Over the first glass substrate, thin film transistors (TFT) 11 are provided near respective intersections of n scan lines G1 to Gn and m signal lines S1 to Sm. The TFT 11 is a nonlinear element (switching element).
Gate lines for the TFTs 11 are connected to the scan lines G1 to Gn. Source electrodes are connected to the signal lines S1 to Sm, and drain electrodes are connected to pixel electrodes. The second glass substrate is disposed so as to face the first glass substrate, and common electrodes are formed all over the surface of the glass substrate, using transparent electrodes made of ITO and the like. Each of the common electrodes is connected to a common electrode drive circuit 12, which defines an electric potential. Liquid crystal 13 is interposed between the common electrodes and the pixel electrodes formed on the first glass.
The scan lines G1 to Gn and the signal lines S1 to Sm are connected to the scan line drive circuit 4 and the signal line drive circuit 3, respectively. The scan line drive circuit 4 applies a high voltage to n scan lines G1 to Gn sequentially, so as to carry out scanning and activate the TFT 11 connected to each of the scan lines G1 to Gn. While the scan line drive circuit 4 scans the scan lines G1 to Gn, the signal line drive circuit 3 outputs gradation voltages corresponding to image data to any of m signal lines S1 to Sm. With this, the gradation voltage is written into the pixel electrodes through the activated TFTs 11. Then, a difference in electric potentials is generated between the common electrodes set at a certain potential and the pixel electrodes where the gradation voltage is applied. With the difference, the volume of light transmission is adjusted.
The backlight 6 disposed at the back side of the liquid crystal display panel 2 is driven by the inverter circuit 7, so as to emit light with a certain level of brightness. Therefore, according to the aforesaid operating principles of the liquid crystal display panel 2, the volume of transmitted light emitted from the backlight 6 is controlled, so that a desirable image display is realized.
The reference gradation voltage generation section 5 illustrated in FIG. 10 supplies the signal line drive circuit 3 with a reference gradation voltage. A reference gradation voltage is applied to the liquid crystal display panel 2, in accordance with a gradation level of image data. FIG. 12 is an explanatory view expressing one example of the relationship between the transmittance of the liquid crystal and the applied voltage. In order to carry out image display with the liquid crystal having the characteristics expressed in FIG. 12, a reference gradation voltage is defined so that the relationship between the gradation level of the image data and the transmittance draws a curve of gamma 2.2 as plotted in FIG. 13, for example. With reference to FIG. 12, one-eighth of the maximum reference gradation voltage is defined as a reference gradation, for example. The rest of the gradation voltages are generated by dividing the adjacent reference gradation voltages. More specifically, all display gradation voltages are defined using divided resistors.
FIG. 14 is an explanatory view illustrating signal waveforms output from the scan line drive circuit 4 and the signal line drive circuit 3 to the scan lines and the signal lines, respectively, in the conventional liquid crystal display. In FIG. 14, the time is set on the abscissa axis. VG1 to VGn are signals output sequentially to n scan lines, by applying a high electric potential (data electric potential) to a single scan line at a time. VD is a signal waveform output to a single signal line, and Vcom is a signal waveform applied to common electrodes. In an example illustrated in FIG. 14, the VD is a signal that changes in signal intensity corresponding to each image data, and the Vcom is a signal that maintains a certain value and does not change with time.
The conventional liquid crystal display and its driving method are described above. When movie display is carried out on the conventional liquid crystal display, display quality deteriorates, for example, after-image emerges. The cause of this problem is a slow response of liquid crystal material. When a gradation of input image data changes, the liquid crystal material cannot respond to the change within one field period. Thus, it is assumed that a few field periods are required to respond. To solve this problem, studies about liquid crystal materials and others have been currently implemented.
Not only the delay of the optical response time, but also an LCD display system itself are pointed out as causes of motion blur in movie display. This issue is brought up at the convention of the institute of electronics information and communication engineers in 1999 (SC-8-1, pp. 207-208), for example. FIGS. 15 and 16 illustrate results of comparing the response time of display light between a CRT and an LCD, in terms of one pixel. FIG. 15 plots the response time of the CRT, and FIG. 16 plots the response time of the LCD. As shown in FIG. 15, the CRT is a so-called impulse-type display, which illuminates for only a few milliseconds after an electronic beam hits fluorescent material on the surface of a tube. As shown in FIG. 16, the LCD is a so-called hold-type display, which holds display light during one field period from the completion of writing data into a pixel to the start of writing next data.
When movie display is carried out in the hold-type display, LCD, a currently displayed image and a previously displayed image are viewed as an overlapped image. The overlap is caused by time integration of visual perception and tracking characteristics of the sight line in the direction of movement. Thus, the motion blur occurs. For preventing the motion blur, several techniques have been proposed as follows. Image data and black data are repeatedly written in the liquid crystal display panel within one field period. Accordingly, a period for carrying out black display (black display period) is set up between one field image display and the next field image display. Thus, a hold time for the display light, that is, an image display period is cut down. In this manner, the display status with the hold-type drive is caused to resemble the display with the impulse-type drive such as the CRT.
FIG. 17 illustrates one example of a so-called black-writing-type liquid crystal display. After writing input image data in one field into a liquid crystal display panel sequentially, black display data is written into all over the screen at the same time, so that the black display over the entire screen is carried out for a certain period of time. With reference to another example illustrated in FIG. 18, black display data is written into the scan lines sequentially, so that the black display is carried out in part of the screen for a certain period of time. Accordingly, the image display period is shortened compared to the one in the conventional hold-type display (disclosed in Japanese Laid-Open Patent Publication No. 127917/1997 and 109921/1999, Tokukaihei 9-127917 and 11-109921). Such a liquid crystal display that has display elements and a shutter is disclosed in Tokukaihei 9-325715, for example. In the liquid crystal display disclosed in this publication, the display elements transform electric image signals to image-display light while continuing the transforming operation for a certain display hold period. Also, the shutter limits the display hold period to a certain time within one field period, in synchronism with the vertical synchronization of image signals.
Furthermore, the following is proposed in Tokukai 2002-123223 and 2002-318569. Only when moving images are displayed on the liquid crystal display panel, the black display is carried out in at least a part of the screen for a certain period of time, so that the impulse-type display is carried out. As a result, the motion blur emerged in movie display can be restrained. In static display, the hold-type display is carried out without setting the black display period. Thus, deterioration of display quality due to flicker and others is prevented.
However, the relationship between the display gradation and the display brightness, that is, so-called gamma characteristics differ between the impulse display mode shown in FIGS. 17 and 18 and the normal hold display mode shown in FIG. 19. The impulse-type display is carried out by producing the black display in at least part of the screen for a certain period of time. In FIG. 20, for example, the full line plots gamma characteristic in the hold display mode. The dotted line plots gamma characteristics when the motion blur is prevented in the impulse display mode shown in FIG. 18. That is, the black display period is inserted into one field period, so that the image display period is shortened. With this, it is recognized that display brightness in a low gradation tends to decrease. Therefore, image-display characteristics vary between movie display with the method shown in FIGS. 17 and 18 and static display with the method shown in FIG. 19. Thus, display quality significantly deteriorates.
In FIG. 21, the full line plots changes in display brightness with time in the hold display mode. The dotted line in FIG. 21 plots changes in display brightness with time when the motion blur is prevented in the impulse display mode illustrated in FIG. 18. As FIG. 21 clearly shows, the ratio in attained brightness between the methods in low gradation display is different from the ratio in attained brightness between the methods in high gradation display. Therefore, it is considered that the different characteristics in display gradation and display brightness (gamma characteristics) between the display methods come from the differences between the response characteristics of the liquid crystal in the low-gradation display and the response characteristics of the liquid crystal in the high-gradation display.
When image display is carried out with the impulse-type display illustrated in FIG. 18, due to the temperature dependency of the liquid crystal as shown in FIG. 22, display brightness in a low gradation tends to decrease as the temperature of the liquid crystal display panel becomes low. That is, gamma characteristics change depending on temperatures of the liquid crystal display panel, so that display quality varies (deteriorates).